Presently, various plastic containers are used for packaging because of their advantages such as light weight, transparency and easiness of molding.
However, because an oxygen barrier property of the plastic containers is lower than those of metal containers and glass containers, the plastic containers have problems in that the contents of the containers degenerate and that the flavor of the contents becomes worse.
For preventing these problems the plastic container walls have a multi-layer structure in which at least one layer is made of a resin having an excellent oxygen barrier property such as an ethylene-vinyl alcohol copolymer. In addition, there are other kinds of containers having an oxygen-absorbing layer for absorbing oxygen remaining in the containers and also oxygen gas penetrating into the containers from the outside. Oxygen gas absorbers (deoxidizer) used for forming the oxygen-absorbing layer include, for example, those mainly containing a reducing substance such as iron powder (see, for instance, Patent Document 1 specified below); or those containing an oxygen scavenger consisting of an ethylenically unsaturated hydrocarbon and a transition metal catalyst (see, for instance, Patent Document Nos. 2 to 4 specified below).
A method in which an oxygen absorber such as iron powder is incorporated into a resin and the resulting resin composition is used as a packaging material for forming the wall of a container used for packaging shows a sufficiently high ability to absorb oxygen gas, but the resulting resin composition has a color hue peculiar to the iron powder. Therefore, said method is limited in its application and cannot be used in the field of packaging in which transparency is required. Moreover, in a method in which an oxygen scavenger consisting of an ethylenically unsaturated hydrocarbon and a transition metal catalyst is used, the ethylenically unsaturated hydrocarbon per se absorbs oxygen gas to ensure the oxygen-barrier properties of the scavenger and thus the amount of the ethylenically unsaturated hydrocarbon to be incorporated into the scavenger should be increased to some extent. However, a problem arises such that the resulting resin composition is insufficient in moldability and transparency if the ethylenically unsaturated hydrocarbon is incorporated into the composition in an increased amount. For this reason, the resin composition is limited in regard to the term during which it can efficiently absorb oxygen gas and thus the composition never satisfies the desired requirements for the long-term storage of the foregoing contents. In addition, coloring of the composition occurs and a bad smell is given off from the composition through the absorption of oxygen.
For solving these problems, the inventors of this invention have found that by the use of a resin composition which comprises a polyolefin resin prepared by polymerizing an olefin having 2 to 8 carbon atoms, a resin other than said polyolefin resin and capable of serving as a trigger for the oxidation of said polyolefin resin, and a specific amount of a transition metal catalyst, the quantity of oxygen gas absorbed by the resin composition can be significantly improved because said trigger resin acts on the polyolefin resin for the initiation of the oxidation thereof so that the polyolefin resin can absorb oxygen gas (see Patent Document 5 specified below).
Patent Document 1: Japanese Examined Patent Publication Sho 62-1824
Patent Document 2: Japanese Un-Examined Patent Publication 2001-39475
Patent Document 3: Japanese Un-Examined Patent Publication Hei 5-115776
Patent Document 4: TOKUHYO Hei 8-502306
Patent Document 5: International Publication 2004/18556 Pamphlet.